1. Field of the Invention
This invention relates to the field of signal transmission and reception over a network, and more particularly to the determination of the strength of a received signal.
2. Description of the Related Art
A data communications network typically transmits information between at least one sender and one receiver. For full duplex communication, the sender is also a receiver and the receiver also a sender. The circuitry combined to perform both functions is typically called a transceiver. All communications networks have some noise present within them, and often the noise varies in magnitude over time, depending upon a number of environmental factors. Thus, all signals sent from a transmitter to a receiver (or from one transceiver to another) within a network must have sufficient magnitude (i.e. strength) such that the receiver is capable of discriminating between the transmitted communication signal and the noise that may be present in the network. Wireless networks tend to be more susceptible to noise than are wired networks. One reason is that wireless transceivers are often mobile (e.g. cellular telephones) with respect to base stations with which they communicate. Additionally, because the signals are transmitted through air, environmental factors such as the distance between the transceivers (i.e. the cellular phone and the base station) and the existence of obstacles such as trees and buildings in the signal path can affect the strength of the received signal, as well as the noise levels that may be present in the wireless network.
Another example of a wireless network is one based on the Bluetooth standard, which is designed to facilitate short-range (i.e. 30 to 60 feet) wireless communication between terminal equipment such as PC""s, laptops, printers, faxes, and hand-held devices such as PDAs (personal digital assistant). The Bluetooth defines a standard by which devices such as the foregoing transmit and receive signals using the ISM (industrial, scientific and medical) radio band of 2.4 GHz. This standard has been established to promote the networking of such devices through compatible transceivers so that they may communicate with one another without need for physical interconnection through proprietary cables. The noise and signal strength issues for a Bluetooth wireless network are analogous to the cellular telephone network, albeit over much shorter distances.
It is known in the art that the real-time measurement of the strength of received signals in networks can provide very useful control information for the network. For example, the signal strength of a signal received from a remote transceiver can be used to notify the remote transceiver to boost or attenuate the output strength of its transmission to compensate for conditions affecting the transmitted signal prior to its reception. The received signal strength (RSS) as measured at a receiving transceiver has also been used to boost or attenuate the transmitted power of its own signal transmissions back to the transceiver that is the source of the measured signal. This ensures that the gain of the return signal is adjusted to offset any noise and/or signal attenuation (or absence thereof) experienced by the received signal. Received signal strength has also been used to determine when communication with a mobile cellular telephone should be handed off to a different cell to provide better transmission, due to, for example, its closer proximity and/or better transmission path.
Numerous methods and circuits have been disclosed for measuring received signal strength, commonly known as an RSSI (Received Signal Strength Indication). Prior such methods and circuits typically involve detecting the peak amplitude of the received signal and averaging the values over some period of time. The implementation of this process typically involves rectifying the input signal using a series of rectifiers, and then sampling the rectified input signal and converting the samples from analog to digital values using A/D (analog to digital) converters. To perform these measurements accurately and over the desired dynamic range of the input signal, a significant number of rectifiers are required and the requisite A/Ds are expensive. Moreover, at the low end of the dynamic range, the peaks may be too small to be discernable over the noise present in the network. Finally, while such RSSI techniques endeavor to provide a measure of the received signal strength at the antenna of the receiver, they do not take into consideration the effect that components internal to the transceiver might have on the signal before the RSSI circuit can perform its measurement.
Therefore, it would be desirable to provide an RSSI method and apparatus that compensates for the variations in the transceiver components that can affect the received signal prior to measurement of the RSS. It would also be desirable to provide an RSSI method and apparatus that is capable of measuring the received signal strength with accuracy and over the requisite dynamic range without the need for a large number of rectifiers and expensive A/D circuits. Finally, it would desirable for the RSSI method and apparatus to have the capability to measure the received signal strength notwithstanding that noise is present in the system that otherwise obscures the received signal from conventional peak detection at low ends of the dynamic range of the received input signal.
One embodiment of the method of the invention for measuring the strength of a received signal involves generating a plurality of DC offsets of increasing value and imposing each of the offsets on a first polarity of the received signal. By determining when each of the offsets is overcome by the amplitude of the received signal, an indication of the strength of the received signal is provided as the greatest of the plurality of offsets overcome by the amplitude of the received signal. In one embodiment, the offsets are determined to have been overcome by the amplitude of the received signal by comparing a first polarity of the received signal for each imposed offset with a second polarity of the received signal to identify when the amplitude of the first polarity exceeds the amplitude of the second polarity.
One embodiment of the apparatus for measuring the strength of a received signal includes a rectifier circuit that further includes an offset circuit having an input for receiving a first polarity of the received signal and producing a plurality of outputs each comprising the one polarity of the received signal added to one of a plurality of unique DC offsets of increasing value. The rectifier circuit further includes a plurality of comparator circuits each having a first input for receiving one of the plurality of outputs of the offset circuit, a second input for receiving a second polarity of the received signal, and an output that is inactive when the amplitude of the signal on the second input is greater than the amplitude of the signal on the first input and active when the amplitude of the signal on the first input is greater than the amplitude of the signal on the second input. The strength of the received signal is thereby indicated by the active outputs of the comparators to be the greatest of the plurality of offsets overcome by the amplitude of the received signal.
In another embodiment of the apparatus, the plurality of DC offsets increases by a step increment equal to the total dynamic range of the received signal in dB divided by the number of the plurality of offsets. In another embodiment, the rectifier circuit is divided into two or more rectifier sub-circuits each having its own offset sub-circuit that generates a portion of the plurality of offsets and a portion of the plurality of comparators. The first polarity of the received signal is coupled to the offset sub-circuit of at least one of the rectifier sub-circuits, and the second polarity of the received signal is coupled to the second input both of which can be amplified by a predetermined gain.
In another embodiment, the received signal is coupled to a variable gain amplifier, the gain of which can be programmed to compensate for the variation in gain supplied by various signal processing components through which the received signal is transmitted before reaching the rectifier circuit. In a still further embodiment, the received signal is down-converted from an RF signal to an IF signal prior to being received by the rectifier circuit.